This image shows a trail left by the BlueWalker 3 satellite over Mexico's National Astronomical Observatory in Sierra de San Pedro Mártir, Baja California.IAU/Observatorio Astronómico Nacional
A team of scientists has been tracking a bright object in the sky. But it's not a star. It's a new type of commercial satellite. Astronomers are trying to understand how its brightness and transmissions will interfere with Earth-based observations of the universe - and what can be done to minimize these effects as more of these satellites are launched.
In a paper published Oct. 2 in Nature, the team reports its first detailed assessment on how the satellite - BlueWalker 3 - could impact astronomy.
"While there is only one BlueWalker 3 satellite, it is one of the brightest objects in the sky, and a harbinger of where low-Earth orbit is heading for sky observers," said co-author Meredith Rawls, a research scientist with the University of Washington's DiRAC Institute and the Vera C. Rubin Observatory in Chile.
Building on initial observations made shortly after its launch, these new results complement the initial understanding of this unusual satellite. The paper includes details of how the satellite's brightness changes over time, as well as the visibility of jettisoned hardware. With companies intending to deploy more commercial satellites in the coming years, this paper highlights the need for pre-launch impact assessments.
"The interference of satellites in astronomy has become an increasingly pressing issue over the last few years," said first author Sangeetha Nandakumar of the University of Atacama in Chile.
BlueWalker 3 was launched into low-Earth orbit on Sept. 10, 2022, by AST SpaceMobile. The craft is a prototype for a planned constellation of more than a hundred satellites for use in mobile communications. Observations made shortly after launch showed that the satellite was among the brightest objects in the sky.
To better understand its impact on astronomy, the International Astronomical Union's Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference, or CPS, initiated an international observing campaign. As part of this initiative, both professional and amateur observations were contributed from across the world from sites in Chile, the United States, Mexico, New Zealand, the Netherlands and Morocco.
"It is exciting that we could incorporate images from many different telescopes and visual observations from highly skilled amateur observers in this analysis," said Rawls. "It's an example of the kind of thing that is possible only when folks from many institutions and with many backgrounds work together with a common purpose."
The CPS is co-hosted by NSF's NOIRLab and the SKA Observatory, an international partnership. The CPS facilitates global coordination of efforts by the astronomical community - in concert with observatories, space agencies, industry, regulators and other sectors - to help mitigate the negative consequences of satellite constellations on astronomy. Its four working groups - or hubs - pursue different projects analyzing different types of interference from satellites and other sources.
"This paper brings together observers from across the globe under the umbrella of the CPS SatHub to better understand the ramifications," said Rawls, who co-leads SatHub. "This is the first peer-reviewed research to quantitatively measure the high brightness of BlueWalker 3 and discuss the impacts on astronomy."
The newly released data show an abrupt increase in the brightness of BlueWalker 3 over a period of 130 days - coinciding with the complete unfolding of its antenna array - followed by fluctuations over the subsequent weeks. Data also showed a relationship between the varying brightness and other factors after unfolding, such as the satellite's height above the horizon and the angle between the observer, the satellite and the sun. The team also used a subset of the observations to calculate the satellite's trajectory over time. Comparing the predicted path with the observations collected, they could evaluate the accuracy of these predictions and observe how its elevation declined over time due to atmospheric drag and other factors.
In addition, they observed the launch vehicle adapter attached to BlueWalker 3 decoupling from the satellite. This component reached magnitude 5.5 in brightness, exceeding maximum recommendations set out by the IAU to avoid the worst impacts of satellites on optical astronomy.
"These results demonstrate a continuing trend towards larger, brighter commercial satellites, which is of particular concern given the plans to launch many more in the coming years," said co-author and CPS scientist Siegfried Eggl of the University of Illinois at Urbana-Champaign. "While these satellites can play a role in improving communications, it is imperative that their disruptions of scientific observations are minimized. This could preferably be achieved through continuing cooperation on mitigation efforts, or, if that is not successful, through a requirement for pre-launch impact assessments as part of future launching authorization processes."
"Besides the effect on visible observations, BlueWalker 3 could also interfere with radio astronomy, since it transmits in radio frequencies close to those that radio telescopes observe in," said Federico Di Vruno, co-director of the CPS. "The novel aspect of BlueWalker 3 is that it uses frequencies that are normally used by terrestrial transmitters."
Observations of BlueWalker 3 will continue, with plans by astronomers to observe its thermal emission later this year. Astronomers will continue to discuss this topic at an IAU symposium this month.
"This is a global issue, since satellites approved by any country are visible in the night sky across the world, highlighting the importance of international coordination," said co-author Jeremy Tregloan-Reed of the University of Atacama and the CPS.
